The invention relates to a colour display tube comprising an evacuated envelope with a rear part and a front part which comprises a display window. The rear part accommodates an electron gun system with which three electron beams whose axes are located in one plane are generated and are focussed by means of a focussing lens field on a display screen which is provided on the inside of the display window. The focussing lens field is produced by a first focussing electrode remote from the display screen and a second focussing electrode facing the display screen of the electron gun system. The first and second focussing electrodes are connected during operation to means for applying a first focussing voltage and a high voltage, respectively. Such a display tube is of a conventional type.
In a cathode ray tube it is often desired to focus an electron beam in, for example, the horizontal direction more strongly than in the vertical direction. This may be necessary, for example, to compensate for astigmatism of the deflection coil or of electron lenses in the tube. This is necessary, inter alia, in colour display tubes having three electron beams located in one plane and a self-convergent deflection coil. Such a deflection coil exerts a converging influence on the separate electron beams in a direction perpendicularly to the plane through the electron beams. The vertical overfocussing which thereby occurs cannot be sufficiently compensated with static means, notably in high resolution colour display tubes, owing to the ever stricter requirements imposed on the definition.
U.S. Pat. No. 4,366,419 describes a main lens construction for a non-integrated in-line colour gun accommodating a system of electrodes for (dynamically) counteracting deflection defocussing. However, this system cannot be used without any further measures in integrated guns.